1. Field of the Invention
The present invention relates generally to ultrasonic dental scalers and more particularly to control circuits for driving the same.
2. Description of the Prior Art
Ultrasonic treatment devices, especially for the removal of tartar from teeth, etc., by means of a magnetostrictive transducer, and a tool selected from a number of such tools secured to it, are generally known.
The common feature of these instruments is that the magnetostrictive transducer, together with the selected, inserted tool or the tool tip, represents a mechanical vibratory system which must be driven at resonant frequency, in order to ensure optimum power output. Many conditions, such as the differences in the length of the tools or of the tool inserts, their wear, the changes in the vibratory system, as well as the mechanical load applied to the tool under operating conditions effect a frequency shift of the vibratory system. Such frequency shifts must be compensated for, either manually by changing the frequency of the ultrasonic generator, or, alternatively, by automatic frequency correction adjustment.
In the known driving and control arrangements which are provided with automatic frequency correction adjustments, either self-excited generators or oscillators are continually frequency-adjusted by means of a feedback signal generated in a coil which is operatively associated with the transducer. Alternatively, additional sensor elements, for example piezoelectric elements, have been provided in the transducer, which supply a signal for the correction adjustment of the oscillator.
Moreover, since dental treatment devices of this kind are driven with high power, the possibility exists that, on careless handling of the device, damage is done to a tooth due to excessive pressure exerted by the tool on the tooth. This danger exists especially in the case of dental treatment devices having automatic frequency adjustment, i.e. tuning means, for the oscillator driving the transducer, because in these devices the transducer continuously operates at a frequency which ensures maximum power output of the transducer.